1. Field of the Invention
The present invention relates generally to an offshore marine platform which is designed to withstand seismic shocks. More particularly, the subject invention is directed to an arrangement wherein a marine platform is supported in suspension from an exterior frame structure resting on the sea floor, and the manner of suspension is designed to compensate for seismic shocks encountered by the marine platform.
2. Discussion of the Prior Art
Some offshore marine platforms resting on the bottom of the sea have to be designed to withstand not only stresses by winds and wave swells but also stresses caused by seismic shocks, this latter factor even being regarded as predominant in areas considered subject to strong seismic phenomena. However, the two types of disturbances proceeding from a wave swell and from a seismic shock respectively manifest themselves in frequency ranges far apart from each other. The result is that a structure which is designed to resist wave swells turns out to be too rigid to resist seismic shocks and that, on the contrary, a structure designed to resist seismic shocks is not sufficiently rigid to resist wave swells.
This has led to a concept of relieving a disturbance reaction by putting into effect a controlled decoupling system for platform structures. More precisely, it has been suggested that the structure be designed rigidly with regard to the action of swells, at the same time arranging integrating linkage parts in the structure which are designed to break following a seismic shock. These parts are specifically designed to break in such a manner as to bring into play flexible interconnecting members held in reserve and arranged to back up the temporary integrating parts.
Zaleski-Zamenhof et al. U.S. Pat. No. 4,152,087 discloses a construction arrangement for an offshore platform of the aforementioned type which provides a controlled decoupling of interconnected component sections of a marine platform structure. The offshore platform structure is designed to be less rigid under seismic shocks while maintaining sufficient overall rigidity to resist the action of wave swells. The coupling system comprises rigid interconnecting linkage parts, such as steel supports, and flexible interconnecting members, such as Neoprene supports, incorporated into the structure. The rigid linkage parts have a structural rigidity sufficient to maintain the overall rigidity of the platform, but are effective to break following a seismic shock. The flexible interconnecting members are held in reserve, and are arranged to back up the rigid intergrating parts. The flexible members have structural characteristics effective to maintain a controlled decoupling of the component sections when the steel supports deform or break.
Marine offshore structures are also relatively well known in the prior art which include a truss type of structure supported by legs extending to the sea floor, and a deck or work platform mounted on the truss structure on which drilling and other types of operations are performed. Templet types of offshore platforms are rigidly anchored to the sea floor by pilings which extend through tubular support legs of the truss structure into the underlying sea floor. When templet offshore structures are located in areas prone to seismic shocks, they must be designed to withstand not only loads imposed by winds and wave swells but also the additional loads of the seismic shocks. In structures of this kind the forces thereon from winds and wave swells are incident through the top of the truss structure, while the seismic shock loads are imposed on the structure through the base frame members thereof.
The prior art designs have often taken a brute force approach to all of the aforementioned imposed forces which has resulted in offshore structures which are relatively massive, incorporating therein tremendous amounts of steel in the truss frame members and, depending upon the design parameters, weighing anywhere from several hundreds of tons to many thousands of tons. The truss frame members are normally tubular in nature to minimize loading on the truss structure from winds and wave swells, and may typically vary from bottom leg members having a forty two inch diameter to top truss members having a diameter as small as ten inches. If the truss members at the top are positioned above expected wave swells, they may be nontubular structural members which are rolled or incorporate flanges therein. In these prior art designs, the deck platform typically rests on a support frame positioned at the top of the truss frame structure, although some recently constructed offshore drilling structures have mounted the deck platform in suspension from the top of the truss structure. An advantage of the latter approach is that the work platform may be floated into the middle of the truss frame structure, and then lifted into place by a cable hoist or hydraulic lift system.